This invention relates generally to glass-ceramic cooktop appliances and particularly to electronic power control systems for such appliances.
Use of glass-ceramic plates as cooktops is well known. Advantages of the smooth surface include pleasing appearance and easy cleanability. Glass-ceramic cooktop appliances using heating units which radiate substantially in the infrared region in combination with a glass-ceramic material which is transparent to such radiation provides the appearance and convenience advantages of conventional thermal conduction type glass-ceramic cooktops plus the additional advantage of greater energy efficiency and improved cooking performance due to a faster response to changes in user selected power settings.
Infrared heating units employ resistance wire elements designed to radiate primarily in the 1-3 micron region of the electromagnetic spectrum. The total output power and watts density parameters for the heating elements in such units is dictated by cooking performance requirements. For domestic appliances the power supply available in the home is generally the line voltage from the local power company. In the United States this is typically 120 and 240 volts. Resistance wire heating elements designed to provide the desired power and watts density at these voltages are constructed of relatively small diameter delicate expensive wire. Significant cost advantages could be enjoyed if the wire diameter could be increased thereby increasing the structural integrity of the wire and making it possible to use a less costly wire material. However, with the power and watts density constrained by cooking performance requirements, any increase in wire diameter must be compensated for by a decrease in voltage. In view of the high current required to provide the desired power, a step-down transformer would be impractical from both a size and cost standpoint.
Thus, to enjoy the benefits of a less costly, more reliable infrared heating unit there is a need for a cost effective practical energy efficient means of reducing the effective voltage applied to the heating units to an effective voltage level less than a domestic line voltage.
Another consequence of increasing wire diameter is that the time required for the wire to reach its radiant temperature is increased. Infrared heating units, at least when operating at or near the maximum user selected power setting, glow brightly. This glow is perceivable by the user through the glass-ceramic cooktop. This glow can be advantageously used to provide prompt visual feedback to the user that the selected unit is operating properly. One such arrangement for rapidly bringing a heating element to its radiating temperature to provide this feedback using commercially available heating elements made of molybdenum disilicide (MoSi.sub.2) or tungsten heating elements is disclosed in commonly assigned U.S. Pat. No. 4,223,498. In that arrangement the unit is driven at the power level associated with the maximum user selectable power setting for a brief period when first turned on, regardless of the actual user selected setting to quickly heat the unit to its radiating temperature.
Since the heating element with the increased wire diameter was designed for operation at a lower effective or RMS voltage, the heat up time can be reduced to an acceptable time by briefly over-driving the heating element at full line voltage. However, the overdrive time must be carefully limited to avoid over-stressing the wire. For example, if the unit is turned off and then on again before it has cooled sufficiently, applying the full line voltage for a time period which has no adverse affect on the wire when heating up from room temperature may damage the pre-heated wire. Use of wire temperature feedback information is impractically costly and complex. Thus, in a system in which the heating elements are designed primarily for operation at voltage levels less than the full line voltage, there is a need for a power control arrangement which can provide an overdrive capability for the elements when turned on but which can adjust the overdrive time to compensate for the past temperature history of the element.
In a multiple element cooktop appliance featuring heating units with elements designed for operation at a voltage stepped down from the normal line voltage, overdriving the elements at full line voltage for short periods of time may draw excessive total current. Household electrical service generally employs a 50 amp breaker in the power circuit for the main kitchen cooking appliances. In a four-unit cooktop appliance for example, the total current is limited by conventional design practice to a maximum level of 35 amps leaving 15 amps for the oven. Since this limit could be exceeded if one or more of the heating units is overdriven depending on the power levels being applied to the remaining units, there is a need for a power control arrangement which can adjust the overdrive power levels to maintain the total current drawn by the heating units within design limits, while still heating the units to radiating temperature relatively quickly to provide the desired visual feedback to the user.
It is therefore an object of the present invention to provide an electric cooking appliance comprising at least one electric heating unit and a power control system which applies an effective voltage level to the heating unit at the maximum user selectable power setting for the unit which is less than the PMS voltage of the domestic power supply so as to accommodate in the appliance heating units designed for maximum steady state operation at a voltage level less than the domestic supply voltage.
It is a further object of the present invention to provide a cooking appliance of the aforementioned type in which the power control system is operative to overdrive the heating units when initially turned on by applying a voltage level higher than the maximum user selectable level for a brief transient heat up period, the duration of which is limited as a function of the elapsed time since the last occurring use of the particular heating unit to avoid overheating a heating unit which is not yet cooled down from its previous use.
It is yet another object of the present invention to provide a cooktop appliance of the aforementioned type in which the power control system is operative to reduce the power level applied to the surface units during operation in the transient heat up period as necessary to maintain the total current drawn by the appliance within predetermined limits.